A Nd—Fe—B sintered magnet (so-called neodymium magnet) comprises a combination of Fe and elements of Nd and B that are cheap, abundant and constantly obtainable natural resources and thus can be manufactured at a low cost and additionally has high magnetic properties (its maximum energy product is about 10 times that of ferritic magnet). Accordingly the Nd—Fe—B sintered magnet has been used in various kinds of articles such as electronic devices and recently adopted in motors and electric generators for hybrid cars.
On the other hand, since the Curie temperature of the above sintered magnet is as low as about 300° C., there is a problem in that the Nd—Fe—B sintered magnet would be demagnetized by heat when heated to a temperature exceeding a predetermined temperature under a certain circumstantial condition in its adopted articles. In addition there is a further problem in that the magnetic properties would be extremely deteriorated by defects (e.g. cracks etc.) or strains in grains of the sintered magnet which are sometimes caused when the sintered magnet is machined to a desired configuration suitable for a particular article.
Therefore, when the Nd—Fe—B sintered magnet is obtained, it is considered to add Dy and Tb which largely improve the grain magnetic anisotropy of principal phase because they have magnetic anisotropy of 4 f electron larger than that of Nd and because they have a negative Stevens factor similar to Nd. However, since Dy and Tb take a ferrimagnetism structure having a spin orientation negative to that of Nd in the crystal lattice of the principal phase, the strength of magnetic field, accordingly the maximum energy product exhibiting the magnetic properties is extremely reduced.
In order to solve this kind of problem, it has been proposed: to form a thin film of Dy and Tb to a predetermined thickness (to be formed in a film thickness of above 3 μm depending on the volume of the magnet) over the entire surface of the Nd—Fe—B sintered magnet; then to perform heat treatment at a predetermined temperature; and to thereby homogeneously diffuse the Dy and Tb that have been deposited (formed into film) on the surface into the grain boundary phases of the magnet (see non-patent document 1).
[Non-patent document 1] Improvement of coercivity on thin Nd2Fe14B sintered permanent magnets (by Pak Kida (in Chinese reading) of Tohoku University Doctor Thesis, Mar. 23, 2000)